HALO timeline.

Halo: Combat Evolved is a 2001 first-person shooter science-fiction video game developed by Bungie Studios and published by Microsoft Game Studios. It is the first game in the Halo series. It focuses on John-117 and the crew of the UNSC Pillar of Autumn who, fleeing from the Fall of Reach, stumble on the Forerunner Halo Installation 04, only to be forced into a desperate battle against the Covenant and the Flood.

Halo 3 is a first-person shooter video game developed by Bungie Studios and published by Microsoft. It is a sequel to Halo 2 and concludes the original Halo trilogy. Halo 3 was released on September 25, 2007 in Australia, Brazil, India, New Zealand, North America, and Singapore; September 26, 2007 in Europe; and September 27, 2007 in Japan all exclusively for the Xbox 360.

Halo 2 is a first-person shooter video game developed by Bungie Studios for the Xbox video game console and is forwards-compatible with the Xbox 360. It is the sequel to Halo: Combat Evolved and features a newly built graphics engine as well as many new gameplay elements. Storywise, Halo 2 develops the struggle between the United Nations Space Command, Covenant, and the Flood during the Human-Covenant War in the fall of 2552.

Halo 4 is the seventh installment in the Halo franchise.[1][6] Set four and a half years after the events of Halo 3, Halo 4 continues the story of John-117 and Cortana, left abandoned on the remains of the UNSC Forward Unto Dawn. The two survivors drift toward the mysterious world of Requiem and the ancient evil within. Halo 4 is the first game developed by 343 Industries, the management team in charge of the Halo franchise, and was published by Microsoft Studios.